Cooperative Screwworm

Eradication Program

Environmental Assessment,

August 2017

Agency Contact:

Denise Bonilla

Veterinary Services

Animal and Plant Health Inspection Service

U.S. Department of Agriculture

2150 Centre Ave Bldg B

Fort Collins, CO 80526

Telephone: 970-494-7317

Non –Discrimination Policy:

The U.S. Department of Agriculture (USDA) prohibits discrimination against its customers, employees, and applicants for 

employment on the bases of race, color, national origin, age, disability, sex, gender identity, religion, reprisal, and where applicable, 

political beliefs, marital status, familial or parental status, sexual orientation, or all or part of an individual's income is derived from 

any public assistance program, or protected genetic information in employment or in any program or activity conducted or funded by 

the Department. (Not all prohibited bases will apply to all programs and/or employment activities.) 

To File an Employment Complaint 

If you wish to file an employment complaint, you must contact your agency's EEO Counselor (PDF) within 45 days of the date of the 

alleged discriminatory act, event, or in the case of a personnel action. Additional information can be found online at 

http://www.ascr.usda.gov/complaint_filing_file.html. 

To File a Program Complaint 

If you wish to file a Civil Rights program complaint of discrimination, complete the USDA Program Discrimination Complaint Form 

(PDF), found online at http://www.ascr.usda.gov/complaint_filing_cust.html, or at any USDA office, or call (866) 632-9992 to request 

the form. You may also write a letter containing all of the information requested in the form. Send your completed complaint form or 
letter to us by mail at U.S. Department of Agriculture, Director, Office of Adjudication, 1400 Independence Avenue, S.W., 

Washington, D.C. 20250-9410, by fax (202) 690-7442 or email at program.intake@usda.gov. 

Persons With Disabilities 

Individuals who are deaf, hard of hearing, or have speech disabilities and you wish to file either an EEO or program complaint 

please contact USDA through the Federal Relay Service at (800) 877-8339 or (800) 845-6136 (in Spanish). 

Persons with disabilities who wish to file a program complaint, please see information above on how to contact us by mail directly or 

by email. If you require alternative means of communication for program information (e.g., Braille, large print, audiotape, etc.) please 

contact USDA's TARGET Center at (202) 720-2600 (voice and TDD). 

Mention of companies or commercial products in this report does not imply recommendation or endorsement by USDA over others not

mentioned. USDA neither guarantees nor warrants the standard of any product mentioned. Product names are mentioned to report

factually on available data and to provide specific information.

This publication reports research involving pesticides. All uses of pesticides must be registered by appropriate State and/or Federal

agencies before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants, and fish and other wildlife—if they are not 

handled or applied properly. Use all pesticides selectively and carefully. Follow recommended label practices for the use and disposal

of pesticides and pesticide containers.
i

Table of Contents

I. Introduction..........................................................................................................1

A. Background .....................................................................................................1

B. Screwworm Biology........................................................................................3

C. Purpose and Need............................................................................................4

II. Alternatives..........................................................................................................5

A. No Action.......................................................................................................6

B. Preferred Alternative......................................................................................6

III. Potential Environmental Impacts.........................................................................8

A. No Action.......................................................................................................8

1. Physical Environment ..............................................................................9

2. Biological Resources ...............................................................................11

3. Human Health and Safety ........................................................................15

B. Preferred Alternative......................................................................................15

1. Physical Environment ..............................................................................16

2. Biological Resources ...............................................................................17

3. Human Health and Safety ........................................................................19

4. Wildlife Protections.................................................................................21

a. Endangered Species Act ....................................................................21

b. Migratory Bird Treaty Act.................................................................22

c. Bald and Golden Eagle Protection Act ..............................................23

5. Environmental Justice..............................................................................23

IV. Potential Cumulative Impacts..............................................................................24

V. Listing of Agencies and Persons Consulted ........................................................27

VI. References............................................................................................................28
1

I. Introduction

A. Background

The New World screwworm, Cochliomyia hominivorax, is a fly of the family Calliphoridae that 

is found in tropical and semitropical regions of North, Central, and South America (USDAAPHIS 2014b). In this document, “screwworm” refers to this species. The screwworm is a 

serious pest of warm-blooded animals such as livestock, domestic and wild animals and, rarely,

human beings, at an estimated average cost of $20 million per year to the United States livestock 

industry (USDA-APHIS 2017a). Because screwworm has the potential to cause significant 

economic losses to animal breeders (Vargas-Terán et al., cited in Fresia et al. 2014), dairy farms, 

and leather production (IAEA/FAO 2000; Vargas-Terán et al. 2005-), it is necessary to plan an 

effective emergency response to ensure that veterinary or medical treatment is done in a timely 

manner and that any risk of spread of released fertile adult flies from the infested host is 

prevented. 

Female flies lay their eggs at the edges of wounds or on mucous membranes of animals, which 

then hatch into larvae (Wyss 2000, USDA-APHIS 2014b, Hall et al. 2016). Flies will not lay 

eggs on dead animals (USDA-APHIS 2014b). As the larvae feed on the flesh of an animal, the 

infested wound becomes enlarged and putrid, attracting more screwworm flies that lay more eggs 

in or near the wound. This condition is called myiasis. Animals with myiasis may display signs 

of discomfort, lethargy, weight loss and depression (USDA-APHIS 2014a, Hall et al. 2016). If 

untreated, a severe case of myiasis may cause the death of the animal within 1 to 2 weeks 

(USDA-APHIS 2014b).

The U.S. Department of Agriculture (USDA), Animal and Plant Health Inspection Service 

(APHIS) has eradicated the screwworm fly from the United States and has participated in 

successful cooperative screwworm eradication programs in Mexico and Central America. The 

United States was declared screwworm-free in 1966, followed by Mexico in 1991, Belize and 

Guatemala in 1994, El Salvador in 1995, and Honduras in 1996 (Wyss 2000). Currently, 

screwworm is found in every South American country except Chile, and in five countries in the 

Caribbean (Robinson et al. 2009, Mastrangelo and Welch 2012, USDA-APHIS 2014b, a). The 

screwworm was eradicated from Panama in 2005 (The Center for Food Security and Public 

Health 2007), and the Darien province in that country is now the northernmost limit of New 

World screwworm (Hall et al. 2016). Sterile screwworm adults are regularly released in the 

Darien Gap to prevent reinvasion from South America, and remains at risk because of Caribbean 

populations (Robinson et al. 2009). 

Although screwworm is usually a pest of large domesticated animals, it was recently identified in 

federally endangered Florida Key deer (Odocoileus virginianus clavium, a subspecies of white-
2

tailed deer). In July 2016, three cases of severe myiasis were found in three domestic animals, 

and in a Key deer, although confirmation did not come until September 2016 (USDA-VS 2016).

The following month, screwworm infestations were reported on several Keys (Delgado et al. 

2016). Many of the Key deer wounds that were infested by screwworms likely were from 

injuries from males battling one another for mates (rutting), causing head and shoulder wounds 

from antlers (FWS 2016b). This outbreak was the first Florida infestation in 57 years (FDACS 

2016c), and the first U.S. infestation in 34 years (FDACS 2016c).

Between October 2016 and January 2017, more than 130 Key deer were killed by screwworm or 

had to be euthanized as a result of screwworm (Association 2017b). The Florida Commissioner 

of Agriculture declared an agricultural emergency, and as a result, the Florida Department of 

Agriculture and Consumer Services (FDACS) established an animal health checkpoint in Key 

Largo, Florida, to check all animals traveling north on Route 1 for signs of screwworm 

infestation to prevent further spread of the fly (FDACS 2016a). In addition, APHIS began 

releasing sterile flies in October 2016 (FDACS 2016a).

In January 2017, a stray dog infested with screwworm was discovered on the mainland, near 

Homestead, Florida, approximately 100 miles northwest of the original infestation. It is unclear 

how the dog arrived in Homestead, but it was the only positive case of screwworm identified in 

the area. Sterile flies were released in the Homestead area as a precaution (Association 2017a). 

In total, more than 188 million sterile flies were released in affected areas of the Florida Keys, 

and more than 17,000 inspections occurred at animal health checkpoints (USDA-APHIS 2017c).

On March 23, 2017, APHIS announced that this effort had successfully eradicated screwworm in 

Florida (USDA-APHIS 2017b). APHIS considers an area to be screwworm-free following

surveillance (trapping flies and visually inspecting animals for signs and symptoms of 

screwworm infestation). No new cases of screwworm have been reported in Florida since 

January 10, 2017. When sterile flies are released, elimination of screwworm is usually achieved 

within three life cycles after the last detection. The flies have on average a 21-day life cycle, and 

they continue to circulate in the area for three weeks beyond each release. In the Keys, APHIS 

completed five life cycles beyond the last positive screwworm detection. Out of an abundance of 

caution, APHIS also released flies in the Homestead area for three completed life cycles. 

APHIS and FDACS will continue passive surveillance to ensure any new findings are quickly 

identified. This surveillance includes veterinarians reporting any suspicious cases, wildlife 

surveillance, concerned citizens that see suspicious wounds on animals or even on a person, and 

continued communication with the parks and the National Key Deer Refuge (USDA-APHIS 

2017b).
3

B. Screwworm Biology

The screwworm’s life cycle length varies with temperature: in warmer temperatures, it can be as 

brief as 18 days, or in cooler areas, as prolonged as 2 to 3 months (Baumhover 2002). Female 

flies lay two or more clutches of up to 400 creamy white eggs at a time (and up to 2,800 eggs in 

a typical 10-30 day lifespan (USDA-APHIS 2017a) in a shingle-like pattern at the edges of 

wounds or body orifices (Laake et al. 1936; Knipling, cited in Novy 1991). Egg batches are laid 

in intervals of a few days (Hightower et al. 1972). Larvae hatch within a day (Matlock and Skoda 

2009), and immediately start to feed on fluids and underlying tissues. As they feed, the wound 

enlarges and deepens due to damage from the larva’s hook-like mouthparts. The odor, serum, 

and blood from the wound attracts additional females to oviposit in the wound. Severe 

infestations may lead to host death (USDA-APHIS 2014b).

Second- and third-instar larvae may be distinguished by their resemblance to a wood screw: they 

have a tapered, cylindrical body with rings of backwardly protruding dark brown spines around 

the body (Figure 2) (Laake et al. 1936), lending the name screwworm (FAO 2008). A second 

diagnostic character is that while other fly larvae are surface feeders, screwworm larvae burrow 

deep below the surface of tissues to feed (FAO 2008, Agriculture 2016b).

Figure 2. Cochliomyia hominivorax screwworm larva (left) (photo by J. Kucharski, USDA ARS)

and adult (right) (photo by Guillermo Fadul, Screwworm Barrier Maintenance Program in 

Panama).

Five to seven days after egg hatch (Matlock and Skoda 2009), mature larvae exit the wound and 

burrow a few centimeters into the ground to pupate (USDA-APHIS 2014a). Depending on 

temperature, pupae mature in as short as 7 days or as long as 60 days (USDA-APHIS 2014a).
4

Adult screwworm flies have a distinctive coloration and pattern: a reddish-orange head, a 

metallic blue or bluish green thorax and three longitudinal dark stripes on the dorsum, or back, of 

the fly (Figure 2) (Laake et al. 1936). Males live from two to three weeks, feeding on flower 

nectar, while females, which feed on flower nectar and animal flesh, live about 10-30 days (The 

Center for Food Security and Public Health 2007). Males can mate within 1 day of emergence, 

females within 3 days (FAO 1992).

Screwworm females may lay eggs in wounds as small as tick or insect bites such as stable or 

horn flies (Alexander 2006, Mastrangelo and Welch 2012), as well as wounds subsequent to 

animal husbandry practices including dehorning, castration, docking, branding, registration 

tagging, and shear cuts (for goats) (Alexander 2006, Robinson et al. 2009, Hall et al. 2016). 

Other sites may include the inner corners of eyes, nasal sinuses, periodontal tissues, rectal and 

genital orifices, navels of newborns, and shedding of antler velvet in deer (Chaudhury et al. 

2010, Mastrangelo and Welch 2012, USDA-APHIS 2014b, FDACS 2016a, Hall et al. 2016).

In addition to visual cues, screwworm females may be attracted to the distinctive odor emanating 

from wounds (FAO 1992, Alexander 2006, Cork and Hall 2007). Volatiles from the bacterial 

action of coliform species residing in the wounds may be responsible for this attraction 

(Baumhover 2002, Cork and Hall 2007, Chaudhury et al. 2010). Other species of flies may also 

infest the wound, and secondary bacterial infections may occur (FAO 1992).

Flies are capable of long-distance flight up to 300 kilometers (Hightower et al. 1965), but 

typically fly only 10 to 20 kilometers in warm, humid environments (OIE, 2013). According to 

Mayer and Atzeni (1993), long-distance dispersal may occur as a result of wind and other 

climatic conditions, habitat factors, and the availability of food and hostss. 

C. Purpose and Need

The discovery of any infested person or animal returning to the United States from infested 

countries requires emergency medical or veterinary treatment to remove any active larvae 

present in open wound tissues. The possible emergence of full grown screwworm fly larvae from 

an infested animal or human poses a risk of fly dispersal to the surrounding community, and this 

potential to spread constitutes a public health threat, an environmental threat, and an agricultural 

threat. Discovery of infested animals is often revealed by veterinary inspection upon arrival, but 

screwworm myiasis in a human may go undetected for several days after the person has returned 

to the United States. Delays in treatment increase the possibility that larvae could emerge and 

reproducing adult flies could escape to the surrounding areas. The opportunistic nature of the 

screwworm flies in these site-specific situations makes it critical that emergency action begin 

immediately to eliminate any pest risk.
5

In response to the elevated threat of infestation of screwworms to livestock and humans in the 

United States, APHIS proposes to participate cooperatively with state agriculture departments in 

localized actions designed to exclude and eradicate screwworms in the United States. The 

emergency nature of these actions to exclude screwworms from being reintroduced to the United 

States requires that all necessary planning documentation be prepared in advance to allow the 

required emergency actions to proceed in a timely manner. Each eradication action will be 

adapted in response to the individual circumstances associated with the detection of persons or 

animals with myiasis. An infested passenger or animal could enter the United States at any 

international airport, border crossing, or marine port. Therefore, actions could occur at any 

location in the country. However, the urgency of response to actual pest risk increases at those 

locations where the screwworm fly has historically been known to breed year-round, particularly

in the southern United States (Arizona, California, Florida, New Mexico, and Texas) where 

average winter temperatures are not cold enough to affect screwworm fly survival. 

APHIS has responsibility for taking actions to exclude, eradicate and/or control agricultural pests 

such as screwworms. The statutory authority for conducting this program in cooperation with the 

state agencies is contained in the Talmadge-Aiken Act (7 U.S.C. 450); and in 21 U.S.C. 111, 

114, 123, and 134. APHIS' authority for regulatory action in this program is based upon 7 CFR 

Part 2.80, which authorizes the implementation of programs to exclude, control, and eradicate 

insect pests that serve as agents of animal disease. The Animal Health Protection Act of 2002, as 

amended (7 U.S.C. §§·8301-8317), provides broad authority for APHIS to prevent the 

introduction into or dissemination within the United States of any pest or disease of livestock (§§ 

8303-8305).

The information presented in this environmental assessment is consistent with requirements in 

the National Environmental Policy Act of 1969 (NEPA), as amended (42 U.S.C. §§ 4321 et 

seq.), NEPA regulations promulgated by the Council on Environmental Quality (40 CFR§§

1500-1508), and APHIS implementing procedures (7 CFR part 372).

II. Alternatives

The two alternatives considered for eradication of screwworms in the United States are: (1) no 

action, and (2) a cooperative screwworm eradication program (the preferred alternative). The 

scope of alternatives is limited necessarily by the need for action; however, flexibility exists 

within the framework of the kinds of actions that are possible within the preferred alternative. 

The possible actions applied to each alternative are described in this chapter and the potential

environmental impacts associated with these alternatives are presented in chapter III.
6

A. No Action

The no action alternative would not involve APHIS in any program to eradicate or exclude 

screwworm flies. APHIS activities under the no action alternative would be limited to 

professional consultation and some coordination in support of activities to eradicate the flies. The 

responsibilities for the eradication of screwworm flies would be given to state and local 

governments with possible assistance from some public health organizations. Selection of the no 

action alternative would be expected to eventually result in additional reintroductions and spread 

of screwworms in the United States. This alternative would have considerable potential for 

adverse effects to public health, to the well-being of livestock, and to the health and survival of 

susceptible species of wildlife. The potential adverse impacts from the selection of this 

alternative would be much greater than from a cooperative screwworm eradication program.

B. Preferred Alternative

Under this alternative, the proposed cooperative screwworm eradication program would be

characterized by a comprehensive program to detect screwworm flies in all life stages, treat 

infested individuals, and, where necessary, release sterile screwworm flies. The program would 

involve the cooperation of APHIS with State and local agencies and departments to eliminate 

any potential pest risk when emergency program action is necessary. APHIS actions related to 

this alternative prevent entry and spread of the screwworm fly in the United States. The costs of 

prevention are much less than would be the control costs in the absence of this effort.

Detection of screwworms requires surveillance of host animals and humans for infection, 

monitoring of fly populations using insect traps, and reporting of cases of myiasis by ranchers, 

veterinarians, and medical personnel. Most detections are expected to be reported through 

veterinary inspections or medical reporting. Trapping with decomposing liver and possibly 

vertical sticky traps may be used if it is suspected that adult flies have emerged at a given 

location. Swormlure-4, the lure used in the screwworm traps, releases a strong odor to attract 

screwworm flies. 

If emergency program action is needed, APHIS will conduct veterinary inspection of domestic 

animals and livestock of foreign origin to detect screwworm myiasis. APHIS will also perform a

thorough pathway study to identify possible sites of fly emergence, including monitoring 

population levels with trapping to determine the extent of the infestation. APHIS’ program 

would require insecticide treatment of all livestock wounds where there is a high risk of 

screwworm introduction.

In the event that humans become infested, the resultant discomfort will probably lead to prompt 

medical treatment. The length of time that elapses before these individuals seek medical 
7

treatment will determine the potential for introduction and spread of screwworms into the United 

States. Earlier medical treatment (preferably immediately upon entry to the United States) would 

result in less chance of fly emergence from a wound.

Program response to the reported entry of an infested person or animal, or of infested wildlife or 

livestock, must be decided on a case-by-case basis. If the screwworm discovery is on a small 

scale, the medical or veterinary treatment of the individual or animal would focus on preventing

further tissue damage and to prevent the emergence from open wound tissue of mature larvae 

that could disperse. The extent of program action after treatment depends upon movement of the 

infested host and development of the larval screwworms in their open wound tissues. APHIS will 

base its program response decisions on similar information in the event of small or large scale 

screwworm infestation in wildlife, as well.

When a case of screwworm myiasis is found several days after a person or animal has returned to 

the United States, APHIS will conduct an epidemiological pathway study to determine the 

possible locations in the United States where emergence of viable larvae from the wound could 

have occurred. This study is done by reviewing the locations where the person or animal has 

been since their entry into the United States. If the screwworm myiasis is detected at the time of 

arrival of the individual into the United States and treated immediately, then further actions may 

not be needed. If the infection is detected at an early stage when the screwworm larvae are only 

in the first instar, then the risk of release of viable screwworms from the infested host is slight 

and further actions may also not be needed to eliminate the risk of screwworm reintroduction 

after treatment. If, however, there has been some larval development in the infested tissue and 

the infested individual has spent some time at various locations in the United States, then further

agency action will be considered.

If the detection of screwworm warrants the use of insecticides to prevent its spread, treatments 

would be made with insecticides registered with the U.S. Environmental Protection Agency 

(EPA) for control of screwworm flies. The treatments would be applied in a timely manner to 

ensure that there is no survival of adult screwworm flies and no movement of flies outside the 

treatment area. The organophosphate, carbamate, and pyrethroid classes of insecticides are 

effective against larval and adult flies (USDA-APHIS 2014b). Doramectin oral doses can be 

administered to livestock for protection (The Center for Food Security and Public Health 2007, 

Mastrangelo and Welch 2012).

A critical method of screwworm control is the sterile insect technique (SIT) (Wyss 2000). 

Control with sterile screwworms, a technique used with great success for the past 61 years, 

involves the mass rearing, sterilization (irradiation of 5-to 6-day old pupae), and ground or aerial 

release of sterilized screwworm flies. The flies are released in sufficient quantities to outnumber 

and outcompete wild screwworm flies in the mating process. When sterile males mate with 

fertile females, the females then lay sterile eggs, which are not viable. Because females only 
8

mate once, the population of flies is progressively reduced (Knipling 1955). After successive 

releases, the wild screwworm population decreases to zero. 

Use of SIT will be considered when APHIS determines that a population of the flies is present or 

may be in the process of developing within the continental United States. Factors to be 

considered include stage of larval development, affected host’s movement history, environmental 

and seasonal considerations, and in the case of humans, time elapsed since U.S. entry, locations 

visited since returning to the United States, and treatment provided (USDA-APHIS 2014a).

The simplest and most rapidly effective release is at ground level. Dispersal by air may be 

considered if the infested area is large, or if other factors make ground release impractical or 

ineffective (USDA-APHIS 2014a). The Florida infestation was treated with ground release of 

sterile flies. Sterile release of flies should continue for at least 3 life cycle periods, or about 9 

weeks in warm conditions after the last positive case or fertile fly detection (USDA-APHIS 

2014a).

The Program would add other methods to the list of approved screwworm eradication program 

methods that may become available in the future that have the same or fewer potential 

environmental impacts. The addition of methods to the program is referred to as adaptive

management. A new chemical treatment would be available for use if it is registered by the 

EPA to control screwworm and poses no greater risks to human health and non-target

organisms than the currently approved treatments.

III. Potential Environmental Impacts

In this chapter, APHIS summarizes the potential impacts of the no action and preferred 

alternatives to the physical, biological, and human environment of the southern portions of 

Florida, Texas, New Mexico, and Arizona. Screwworm thrives in tropical and subtropical 

environments, and can survive winters in these states (Novy 1991).

A. No Action

The no action alternative would not involve APHIS in any program to eradicate or exclude 

screwworm flies. The responsibilities for the eradication or suppression of screwworm flies 

would be given to state and local governments. Although state and local governments might 

successfully eradicate and exclude screwworms under certain circumstances, their access to 

useful resources on an emergency basis is more limited than APHIS. For example, the SIT used 

against screwworm flies is strictly a Federal or international effort that would not be directly 

available to state or local governments. Selection of the no action alternative would be expected 

to eventually result in the reintroduction of screwworms to the United States and eradicated areas 
9

in Mexico and Central America. The rate of dispersion from the site of introduction depends 

upon location and climatic conditions at the time, but dispersion could eventually include the 

historic limits in North America. The screwworm fly was historically established in the southern 

United States and dispersed northward as far as the Canadian border annually. Prior to 1962, the 

state of Texas, which accounted for the majority of screwworm infestations, estimated that up to 

1 million livestock animals were infested in a given year. From then until the late 1970s, 

screwworm infestation fluctuated with weather and other factors, from the hundreds to thousands 

(Novy 1991). Inaction or inadequate action would have considerable potential for adverse effects 

to public health, to the well-being of livestock, and to the health and survival of susceptible 

species of wildlife. The potential adverse impacts from the selection of this alternative could be 

much greater than from a cooperative screwworm eradication program.

1. Physical Environment

The affected area includes the historic range of the screwworm in the central and southern United 

States (James 1947). Most important to the incidence and severity of potential screwworm 

infestations are climatic conditions. Screwworm populations are most abundant in hot and humid 

areas (Vargas-Terán et al., 2005). Due to eradication efforts, screwworm has not occupied the 

extent of its potential range in decades; however, the northward trend in warmer temperatures

could lead to an expansion of the screwworm’s range northward in the United States. 

National, state, county, and city parks, as well as National Wildlife Refuges, are located within 

the geographic area described for this assessment. An example of the parks within the affected 

environment are located in Table 1. Parks are considered mixed-use and are used for recreational 

purposes and preservation of animal habitats and ecological resources. National Wildlife Refuges 

are managed by the U.S. Fish and Wildlife Service (FWS) and are areas set aside to conserve 

fish, wildlife, and plants. 

Table 1. Several parks and wildlife refuges in the affected area

State Park or Refuge

Arizona Buenos Aires National Wildlife Refuge

Cabeza Prieto National Wildlife Refuge

Grand Canyon National Park

Leslie Canyon National Wildlife Refuge

Saguaro National Park

San Bernadino National Wildlife Refuge

California Coachella Valley National Wildlife Refuge

Death Valley National Park

Joshua Tree National Park

San Diego National Wildlife Refuge

Seal Beach National Wildlife Refuge
10

State Park or Refuge

Florida Biscayne National Park

Everglades National Park

National Key Deer Wildlife Refuge

Key West National Wildlife Refuge

Ten Thousand Islands National Wildlife Refuge

New Mexico Carlsbad Caverns National Park

San Andres National Wildlife Refuge

Texas Big Bend National Park

Guadalupe Mountains National Park

Laguna Atascosa National Wildlife Refuge

Lower Rio Grande National Wildlife Refuge

Soil plays a key role in determining the capacity of a site for biomass vigor and production in 

terms of physical support, air, water, temperature moderation, protection from toxins, and 

nutrient availability. Soils also determine a site’s susceptibility to erosion by wind and water, and 

flood attenuation capacity. Soil properties such as temperature, pH, soluble salts, amount of 

organic matter, the carbon-nitrogen ratio, numbers of microorganisms and soil fauna all vary 

seasonally, as well as over extended periods of time (USDA-NRCS 1999). Soils are organized 

into four levels of classification, the highest being the soil order. Soils are differentiated based on 

characteristics such as particle size, texture, and color, and classified taxonomically into soil 

orders based on observable properties such as organic matter content and degree of soil profile 

development (USDA-NRCS 2010). The Natural Resources Conservation Service (NRCS) 

maintains soil maps on a county level for the entire United States and its territories. Under the no 

action alternative, soils in the program area would not be disturbed.

The Clean Air Act (CAA) requires the maintenance of National Ambient Air Quality Standards 

(NAAQS). The NAAQS, developed by the EPA to protect public health, establish limits for six 

criteria pollutants: ozone (O3), nitrogen dioxide (NO2), carbon monoxide (CO), sulfur dioxide 

(SO2), lead (Pb), and inhalable particulates (coarse particulate matter [PM] greater than 2.5 

micrometers and less than 10 micrometers in diameter [PM10] and fine particles less than 2.5 

micrometers in diameter [PM2.5]). The CAA requires states to achieve and maintain the NAAQS 

within their jurisdiction. Each state may adopt requirements stricter than those of the national 

standard and each is also required by EPA to prepare a State Implementation Plan containing 

strategies to achieve and maintain the national standard of air quality within the state. Areas that 

violate air quality standards are designated as non-attainment areas for the criteria pollutant(s), 

whereas areas that comply with air quality standards are designated as attainment areas. Nonattainment areas are typically associated with large metropolitan areas with many mobile (e.g., 

vehicle) and stationary (e.g., power plants and factories) sources. Other than emissions from 

mobile sources, crop farming emission sources are not specifically regulated nationwide under 

the CAA. The degree to which emissions from farming practices (such as prescribed burning) are 

allowed are location-specific within each State Implementation Plan (US-EPA 2011). The no 
11

action alternative will not impact air quality, and any vehicle use associated with this alternative 

is minor compared to background use of vehicles, particularly in metropolitan areas.

The principal law governing pollution of the nation’s water resources is the Federal Water 

Pollution Control Act of 1972, better known as the Clean Water Act (CWA). The Act uses water 

quality standards, permitting requirements, and monitoring to protect water quality. The EPA 

sets the standards for water pollution abatement for all waters of the United States under the 

programs contained in the CWA, but, in most cases, gives qualified states the authority to issue 

and enforce permits. Drinking water is protected under the Safe Drinking Water Act of 1974 

(Public Law 93-523, 42 U.S.C. 300 et seq.). 

Surface water in rivers, streams, creeks, lakes, and reservoirs supports everyday life through the 

provision of water for drinking and other public uses, irrigation, and industry. Surface runoff 

from rain, snowmelt, or irrigation water can affect surface water quality by depositing sediment, 

minerals, or contaminants into surface water bodies. Surface runoff is influenced by 

meteorological factors such as rainfall intensity and duration, and physical factors such as 

vegetation, soil type, and topography. The no action alternative will not change patterns of 

surface water, ground water, or drinking water from current patterns.

2. Biological Resources

Vegetation

Diverse vegetation characterizes the affected environment. APHIS identified a representative 

selection of plant species for regions in Arizona, California, Florida, New Mexico, and Texas in 

Table 2. The no action alternative will not affect plant communities in the affected area.

Table 2. Representative vegetation in the affected area (Society 2017, USDA-APHIS 2017a, 

Association No Date)

State Region Vegetation

Arizona Southern Prickly pear, cholla, creosote bush, mesquite

California

Coast live oak, Fremont cottonwood, California buckeye, 

mission manzanita, western columbine, California wax 

myrtle, sand dune sedge, quaking aspen, Santa Lucia 

bush mallow, succulent species

Florida Southern coastal 

flatwoods

Slash and longleaf pines; cabbage palms and live oaks; 

understories of saw palmetto, gallberry and grasses

Everglades and Keys Bald cypress, mangroves, willows, sawgrass, pickleweed, 

buttonbush and maidencane
12

State Region Vegetation

New Mexico Southern

Desert grasses and shrubs such as mesa dropseed, 

creosote bush, sagebrush, yuccas, blue grama grass and 

sideoats grama grass, junipers, pinyons, and ponderosa 

pine

Texas Gulf prairies and 

marshes

Oaks, elms, tall bunchgrasses, saltgrass, pricklypear, 

sedges and rushes

Wildlife

A wide variety of mammals, reptiles, amphibians, birds, and fish inhabit the affected area. Some 

representative wildlife are listed in Table 3. If the no action alternative is selected, some animals 

may become infested with screwworm, and population levels could decline significantly. The 

Florida Key deer infestation that ensued in 2016 resulted in the deaths of more than 130 deer, 

illustrating the potential of screwworm to quickly impact wildlife populations.

Table 3. Representative wildlife in the affected environment

State Mammals Birds

Reptiles and 

Amphibians Fish References

Arizona Mule deer Quail Rattlesnake Trout (USDA-NCRS 2006)

Desert bighorn 

sheep Kingsnake

Largemouth 

bass (Department 2016)

Antelope Crappies

Javelina Catfishes

Coyote Yellow perch

Arctic grayling

California Bats Loons Salamanders Crappies (Wildlife 2016a)

Squirrels Grebes

Frogs and 

toads Bullheads

(University of 

California 2017)

Mice, rats and 

voles Albatrosses Tree frogs Trout

Foxes Herons

Snapping 

turtles California tilapia

Wolves Boobies Tortoises Salmon

Raccoons Cormorants Geckos

Coastal three

spine 

stickleback

Storks and ibises Gila monsters

Humboldt 

sucker

Pit vipers Sculpins

Florida

White-tailed 

deer Woodpeckers Alligators

Largemouth 

bass (USDA-NCRS 2006)

Feral hog Mourning dove Turtles Black crappie

Bobcat Bobwhite quail

Red drum and 

blackdrum

Wood storks Sea trout
13

State Mammals Birds

Reptiles and 

Amphibians Fish References

Raccoon White ibis Tarpon

Skunk Egrets Sheepshead

Otter Herons Snook

Squirrel Snipe

New Mexico Mule deer Scaled quail

Eastern 

snapping turtle Gila trout (USDA-NCRS 2006)

White-tailed 

deer Mourning dove

Rio Grande 

cooter

Rio Grande 

cutthroat trout (Society 2017)

Antelope

Western 

painted turtle (Fish 2016)

Javelina Alligator lizards

Mountain lion Leopard lizards

Cougar Geckos

Foxes Gila monsters

Bobcat

Sonoran 

whipsnake

Badger

Western 

diamond 

rattlesnake

Toads and 

frogs

Salamanders

Texas Armadillo Cormorants Cottonmouths Black bass

(Association No 

Date-a)

Bats American coot Copperheads Temperate bass (Wildlife 2016b)

Bear Egrets Alligators Catfishes (Wildlife No Date)

Bison Herons Bullheads

Coyote Killdeer Crappies

Beaver Woodpeckers Sunfishes

Deer

Ruby-crowned 

kinglet Carps

Foxes Meadowlarks Minnows

Gophers

Brown-headed 

cowbird Gars

Prairie dogs

Red-winged 

blackbird

Blackdrum and 

reddrum

Javelina Snook

Sheep Seatrout
14

Agricultural Production

The following is a brief summary of key agricultural crops in the affected states. Arizona’s chief 

commodities are cotton, lettuce, sorghum, spring wheat, broccoli, cauliflower, chile peppers, 

honeydew and cantaloupe (USDA-NASS 2016d). California’s number one crop is almonds, 

followed by grapes, strawberries, English walnuts, and hay. Oranges and grapefruit production 

are also important in California (USDA-NASS 2016a).

Chief crops in Florida are oranges, strawberries, bell peppers, grapefruit, and peanuts, in that 

order. Other citrus fruits such as tangerines and tangelos also contribute to the state’s economy 

(USDA-NASS 2016b). Much of the land in the affected area of southern California is under 

irrigation, and is managed for crops such as rice, sugar beets, cotton, and grain (USDA-NCRS 

2006).

The most important crops in New Mexico are pecans, hay, cotton, and onions (Agriculture 

2016c). Major commodities in Texas are cotton, grain corn, sorghum, and wheat (USDA-NASS 

2016c). The no action alternative will not have any impact on agricultural production in Arizona, 

California, Florida, New Mexico, or Texas.

Livestock Production

For the purposes of this EA, livestock refers to cattle and calves; hogs and pigs; sheep and lambs; 

and goats, and also includes milk, wool, and mohair production. Cattle and calves refers to all 

cattle and calves, whether raised for beef or for dairy. 

Of the states in the affected area, Texas has the largest livestock industry (Table 4). In 2015, 

California led the nation in milk production with 40.9 billion pounds (lb), followed by Texas 

with 8.27 billion lb, New Mexico with 7.8 billion lb, Arizona with 4.7 billion lb, and Florida 

with 2.5 billion lb (USDA-NASS 2012, Agriculture 2016a, Agriculture 2016c, FDACS 2016b, 

USDA-NASS 2016d).

Table 4. Livestock production in the affected area in 2015 (Agriculture 2016a, Agriculture 

2016c, FDACS 2016b, USDA-NASS 2016d)

State Cattle and Calves Hogs and Pigs Sheep and Lambs Goats

Arizona 0.88 million 139,000 150,000 64,000

California 5.1 million 95,000 330,000 128,400

Florida 1.7 million 18,000 no data 47,400

New Mexico 1.38 million 1,500 90,000 11,000

Texas 11.7 million 880,000 735,000 928,000
15

Wool is another valuable livestock commodity. In 2015, California produced 2.85 million lb of 

wool, Texas produced 1.95 million lb, Arizona produced 680,000 lb, and New Mexico produced

648,000 lb. Mohair, from Angora goats, is produced in Arizona (120,000 lb), and Texas 

(580,000 lb) (USDA-NASS 2012, Agriculture 2016a, Agriculture 2016c, FDACS 2016b, 

USDA-NASS 2016d). All of the livestock production facilities in states within the proposed 

program area are vulnerable to screwworm infestations. Under the no action alternative, APHIS 

would not be able to treat outbreaks; this role would fall to state and local authorities and 

livestock producers. Subsequently, livestock producers have a greater likelihood of suffering 

losses under the no action alternative than under the preferred alternative.

3. Human Health and Safety

In rare instances, humans may become infested with screwworm. However, the discomfort 

associated with an infestation would usually lead the individual to seek prompt medical attention, 

and risk of spread of the infestation would likely be minimal.

If the no action alternative is selected, it is possible that livestock may become infested. Because 

livestock are raised primarily for meat or for commodities such as milk and wool, if infestations 

are large enough to cause serious animal losses, meat and milk prices may rise to accommodate 

shortages, which negatively impacts consumers. Meat as a dietary source of protein could be 

replaced by other food items, but this may be challenging for some populations. This situation is 

both stressful for producers suffering loss of livestock and associated income, and for consumers 

who may no longer be able to afford these products.

B. Preferred Alternative

The objective of the screwworm eradication program is to prevent entry of screwworms and 

eradicate any introduced screwworm flies released into the United States from an infested person 

or animal. The proposed program involves (1) detection of screwworm flies in all life stages, (2) 

treatment of infested individuals, (3) insecticide applications to eliminate screwworm flies and, if 

necessary, (4) the release of sterile screwworm flies. 

Program monitoring of screwworm fly populations involves detection and reporting by ranchers, 

veterinarians, and medical personnel, trapping, and surveillance of host animals and humans. 

Trapping may be used if APHIS suspects that adult flies have emerged at a given location. 

An infested human or animal should be treated as soon as possible by medical, veterinary, or 

wildlife personnel in order to prevent further tissue damage and emergence from the wounded 

tissue of mature larvae. Adherence to proper disposal of wastes from the medical and veterinary 

procedures ensures that there are no environmental risks and eliminates the potential risk of 
16

spread from screwworms that could emerge from untreated wounds. This treatment and proper 

disposal are critical to eliminate the public health risks, animal health risks, and risks to

nontarget wildlife associated with screwworm fly infestations.

In addition to basic veterinary treatment, application of a formulated permethrin, coumaphos, or 

doramectin product may be used to treat external wounds on domestic animals and livestock to 

control and/or prevent screwworm infestations (USDA-APHIS 2014b). These formulations are 

registered with the EPA for this use. Program personnel are trained in the proper application 

procedures and safety precautions for these insecticides. Applications of these formulations are 

effective against screwworm flies and are not repellent to the flies. Although other insecticide 

formulations may effectively control screwworm flies, they may be more toxic to the infested 

host, be more irritating to the wound tissue, have repellent qualities to screwworm flies, or lack 

current EPA registration for this type of treatment. As part of the planned mitigation measures 

for the program, the program will also implement appropriate pesticide storage and disposal 

procedures for insecticides. The rapid response required for screwworm emergency efforts 

makes it important that insecticide applications be considered in a timely manner to all sites 

where there is high risk of emergence of screwworms from wounds of infested individuals.

Release of sterile flies is an important component of the program for eliminating screwworm 

populations. The sterile release program is discussed in detail in Chapter 2, Alternatives, under 

the preferred alternative.

1. Physical Environment

Preferred alternative actions associated with the screwworm program have some potential 

environmental impacts, but the adverse impacts from these actions are generally minimal and of 

short duration. These potential environmental impacts are discussed below.

Spillage of insecticides is a potential problem with respect to environmental contamination, 

although following best management practices will reduce the likelihood of this occurring. 

Ensuring that pesticide applicators are well-trained to handle spills will help mitigate potential 

environmental impacts that could occur as a result of a spill.

Permethrin is a synthetic pyrethroid that targets the insect nervous system. It may be used in 

mosquito treatment, used in food and feed crops, and on ornamental lawns, and on livestock and 

pets, structures, and buildings. If permethrin is spilled, it is fairly immobile in soil, where it is 

usually degraded by microorganisms. Sunlight may break down permethrin in the top layer of 

soil or surface water, but adheres well to sediments and persists for a relatively long time. 

Permethrin has low water solubility (EPA 1996, 2009), and will, therefore, not significantly 

impact groundwater (Toynton et al. 2009). 
17

Coumaphos is a broad-spectrum organosphosphate chemical used to control arthropod pests on 

livestock by means of wettable powders, emulsifiable liquids, dusts, and flowable and ready-touse products. As with permethrin, coumaphos is immobile in soil, but rapidly degraded by 

sunlight in water. It can, however, leach from soil into groundwater. Dip vats are used by APHIS 

to treat cattle near the United States-Mexico border, primarily to control ticks. Coumaphos 

residues in cattle dip vats may be degraded by bacteria that contaminate the vats (EPA 1996). 

Doramectin is a fermentation product of a specific strain of Streptomyces avermitilis structurally 

similar to abamectin and ivermectin in the macrocyclic lactone chemical class. Doramectin 

affects gamma-aminobutyric acid (GABA)-sensitive neurons, which can lead to neurotoxicity, 

expressed by tremors, ataxia and gait abnormalities in mammals. The only volatile component of 

doramectin formulations is isopropyl alcohol, which is emitted in very small quantities during 

application. Its aqueous solubility is low, and it is rapidly broken down by sunlight in water. 

Doramectin residues strongly sorb to feces, and residues may be concentrated over time, 

although total quantities are thought to be very small (FDA 1997b, a). Through the excretion of 

doramectin in livestock and other animal waste, doramectin may contribute to rainfall runoff

and, therefore, could be a source of pollution to soil, surface water and groundwater (Gil-Diaz et 

al. 2011).

The SIT program has minimal impact on the environment except to the target insect, the 

screwworm. Irradiated screwworm flies are sterile, but they are not radioactive. 

The use of small aircraft to disperse sterile flies will release some pollutants such as carbon 

dioxide, nitrogen oxides, carbon monoxide, sulfur oxides, and water vapor; however, since 

agricultural aviation is currently used to treat crops, it is unlikely that planes used to release flies 

would add significantly to pollutant loads. The same is true for the use of road vehicles to move 

to treatment points, or to release sterile flies by ground.

2. Biological Resources

Since livestock are important to United States agriculture for commodities such as meat, milk 

and dairy products, and wool, prevention and treatment of screwworm is crucial. Given the 

history of screwworm in the United States before its eradication in 1966 (Wyss 2000), and its 

detrimental effect on livestock populations, preventing outbreaks from reaching livestock is 

important.

The preferred alternative should lead to overall benefits to livestock and wildlife. If animals 

become infested with screwworm, veterinary treatment with chemicals such as permethrin, 

doramectin or coumaphos may be used. The use of insecticides occurs primarily with livestock. 

However, with the discovery of screwworm on National Key Deer Refuge in Florida, FWS
18

implemented a program to locate infested deer and administer medication, treat wounds by 

wildlife veterinarians, and to euthanize deer as appropriate. FWS prophylactically treated Key 

deer with oral doses of the drug doramectin. In addition, twenty-seven stations were constructed 

to allow deer to self-medicate by visiting the stations, and more than 3,000 doses of doramectin 

were administered (Service 2016).

Exposure to these chemicals can be minimized by management of treated animals. Following 

treatment for screwworm or for other parasites, livestock are usually kept penned for a specific 

refractory period, to avoid contact with humans and wildlife (Zoetis 2013b). Chemical residues 

may be transferred from a treated animal to an untreated animal by activities such as grooming 

(Floate et al. 2005), but these quantities are expected to be small. Wildlife and livestock may 

come into contact with each other in locations such as foraging areas and sources of water, but 

again, transfer of pesticide residues are likely to be very small.

When any pest control program is implemented, the effect on nontarget organisms also must be 

considered. Coumaphos and doramectin are administered directly to animals to kill parasites, and 

both are excreted in the feces of the treated animals. Gil-Diaz et al. (2011) noted that “as 

emerging contaminants, feces-borne drugs are suspected of causing adverse effects in both 

humans and wildlife.” Residues of veterinary parasiticides such as doramectin can persist in the 

feces of treated animals for several months. For example, Gil-Diaz et al. (2011) found that 

doramectin could be detected in soil that had been amended with pig manure up to 7 months.

This long persistence in soil could negatively affect soil biodiversity, and successive applications 

of manure contaminated with doramectin could result in accumulation that could adversely affect

soil fauna, such as decomposers (Gil-Diaz et al. 2011, Wall and Beynon 2012). Doramectin can 

also slow down dung degradation and have deleterious effects on nematodes in the decomposer 

community. It can also have potential negative consequences for microorganism and vertebrate 

predators of insects (Floate et al. 2005, Wall and Beynon 2012). 

A study of the effect of coumaphos residues in manure demonstrated that nontarget dung beetles 

were negatively affected (Lumaret and Errouissi 2002). Coumaphos is highly toxic to birds, and 

moderately to highly toxic to fish, and highly toxic to aquatic invertebrates on an acute basis

(EPA 1996). Coumaphos used as a miticide in honey bee hives is also implicated as a factor in 

colony collapse disorder in bees (Johnson et al. 2010).

Studies of permethrin effects are somewhat varied. Shleier and Peterson (2010) used caged house 

crickets as surrogates for medium-sized nontarget insects following a mosquito spray application

to assess nontarget toxicity. They showed low mortality in the crickets, and suggested that the 

ultra-low volume mode of permethrin application likely resulted in lower concentrations of 

permethrin reaching the crickets, and, therefore, less exposure. However, a study of the effect of 

permethrin and its metabolites excreted in feces following veterinary treatment demonstrated 
19

mortality to nontarget insects (Lumaret and Errouissi 2002). In addition, when permethrin 

contaminates surface water, it may produce toxic effects in a range of aquatic invertebrates such 

as chironomid midges; the larvae of caddisflies, dragonflies, damselflies, and mayflies; and 

beetles and true bugs. Exposure can take place directly via sprays, or indirectly by water runoff , 

spray drift and erosion (Antwi and Reddy 2015). Permethrin is considered to be highly toxic to 

bees and fish, but low in toxicity to birds (Toynton et al. 2009).

In tests of selected fruits and vegetables, less than 1 percent of the sources contained permethrin

(Toynton et al. 2009). Doramectin is not expected to affect terrestrial plant growth, and tests of 

exposure of the drug to freshwater algae showed that it was not acutely toxic to the algae (FDA 

1997b). In general, organosphosphates have few negative nontarget effects on plants, but one 

study noted that the chemical caused an increase in oxidative damage in the estuarine macroalga 

Ulva pertusa (Schweikert and Burritt 2012).

APHIS has determined sterile insect technique to be safe for use in all habitats, including 

endangered and threatened species habitats. In the course of their release, sterile flies may serve 

as food for fly predators. With regard to effects on endangered and threatened species, APHIS 

will consult with the FWS and/or the National Marine Fisheries Service (NMFS) for each 

individual eradication effort, and will comply with all protection measures stipulated in that 

consultation and mutually agreed on with FWS and NMFS.

3. Human Health and Safety

The potential consequences of implementing this program to the health of humans are minimal. 

It is possible, but not likely, that humans may become infested with screwworm. In that event, 

prompt medical treatment should be sought. The preferred alternative will be a benefit to human 

health because humans who own livestock or pets will be less distressed or worried about the 

potential for their animals to become infested, and if infested, that they will be treated promptly.

Humans with potential exposure to doramectin, coumaphos, and permethrin include program 

workers and the general public. Program workers are the most likely people to be exposed to 

chemicals. Under normal conditions with proper functioning personal protective equipment 

(PPE) and proper worker hygiene, occupational exposure to these chemicals is not expected. 

Accidental exposure may occur during applications. However, these types of accidental 

exposures are minimal with well-trained applicators following the label safety precaution (Zoetis 

2013b, a). Risk from occasional accidental exposure is minimized by adequate training of 

workers. Occupational exposure for workers treating wildlife (such as the Key deer in the 2016 

outbreak) would be similarly very low. Other federal agencies such as FWS would handle 

treatment of wildlife.
20

For the general public, potential direct contact exposure to permethrin, doramectin, or 

coumaphos is unlikely because the treatment area should be secured, with no public access to 

treated livestock during the application period. Therefore, dermal, inhalation or accidental 

ingestion exposure is not expected. 

Doramectin has low toxicity in mammals because GABA is found only in the central nervous 

system of mammals and is protected by the blood-brain barrier (FAO/WHO 2006). Although 

doramectin is a hazard to humans at high doses, risks to program workers from doramectin via 

oral, inhalation, and dermal exposure is minimized by adherence to label directions regarding 

proper use of PPE. Overall, the preferred alternative use of doramectin to control screwworms is 

expected to pose minimal risks to human health. 

Dietary exposure to meat from doramectin-treated cattle or swine is unlikely because APHIS will 

adhere to the time requirement on the insecticide label for withdrawal time, and discontinue use 

of doramectin 45 days for cattle, and 35 days for swine prior to slaughter. These withdrawal time 

periods allow doramectin residues to decrease to below the tolerance levels (a liver tolerance of 

100 ppb in cattle, and 160 ppb in swine) (FDA 1997a, b). Consequently, risks to the general 

public from dietary consumption of doramectin in cattle or swine meat is expected to be 

negligible.

For applicators, the primary route of coumaphos exposure is through the skin. To minimize 

exposure, applicators are restricted in the number of animals they can treat in one day. PPE is 

generally required by the pesticide use label, including gloves, and sometimes respirators, 

depending on the formulation and application rate. EPA believes that exposure of coumaphos to 

persons entering treatment sites after application is minimal because most coumaphos is applied 

directly to livestock, and applicators usually do not contact the animals directly after the 

application procedure. Coumaphos is classified as not likely to be carcinogenic in humans.

Humans may be exposed to coumaphos in drinking water since coumaphos wash-off from 

treated cattle can come into contact with and be adsorbed onto manure as well as transported as 

runoff, reaching surface and ground water sources of drinking water (EPA 2016). Dietary 

exposure to coumaphos from treated cattle or from milk is considered to be low (EPA 1996).

In humans, permethrin has low dermal toxicity and absorption and low inhalation toxicity, but it 

may cause skin irritation (Toynton et al. 2009). Mammalian prenatal development and 

reproductive studies show low chronic toxicity. EPA considers permethrin to be a possible 

human carcinogen (EPA OPP, 2003). Applicators for the screwworm eradication program are 

likely to have minimal exposure because of the use of PPE. The general public are likely to have 

even less exposure.
21

Workers in the USDA production labs rearing sterile flies are subject to a safety protocol, which 

lowers risk of exposure to radiation to a minimal level. Risk from released flies to the general 

public will not be any different than that from wild flies.

4. Wildlife Protections

a. Endangered Species Act

The Endangered Species Act of 1973 (ESA) as amended (16 U.S. C. 1531 et seq.) requires all 

Federal departments and agencies to consult with the FWS and/or NMFS to ensure that any 

action that they authorize, fund, or carry out is not likely to jeopardize the continued existence of 

an endangered or threatened species or result in the destruction or adverse modification of its 

critical habitat (16 U.S.C. 1536(a)(2)). Most agency actions related to cooperative screwworm 

eradication efforts are expected to occur in highly disturbed areas that would not be suitable 

habitats for endangered and threatened species. The use of SIT has been determined to be 

compatible for use in habitats of endangered and threatened species and could be applied 

effectively at some locations if necessary. APHIS will consult with FWS and/or NMFS regarding 

endangered and threatened species for each individual eradication effort and will comply with all 

protection measures stipulated and mutually agreed upon in that consultation.

Section 7 of the ESA and its implementing regulations require Federal agencies to ensure their 

actions are not likely to jeopardize the continued existence of listed threatened or endangered 

species, or result in the destruction or adverse modification of critical habitat.

(1) Potential Effects of Screwworms on Threatened and Endangered Species

Other than Key deer, no other listed species have been reported or detected with screwworm 

infestations.

(2) Potential Effects of Screwworm Eradication Programs on Threatened and 

Endangered Species and Critical Habitat

Traps using Swormlure-4 or rotten liver to attract the screwworm flies would not likely affect 

listed species or critical habitat other than potential disturbance of listed animals in an area. 

Although traps may capture other arthropods, APHIS anticipates that screwworm flies will be the 

primary organism collected. Listed arthropods would not be expected to be attracted to the traps, 

but APHIS will avoid placement of traps in the habitat of listed arthropods. 

The use of sterile screwworms is the method most commonly used for eradication programs, and 

is compatible for use in habitats of endangered and threatened species. In the case of Florida, 

ground releases of sterile screwworms were widely used in the habitat of the Key deer and other 
22

listed species. Ground release of sterile flies resulted in little disturbance to listed species or 

critical habitat in the area. However, aerial releases of sterile flies could cause some temporary 

disturbance to some species because of the noise of the plane. 

The application of a formulated permethrin or coumaphos product to treat external wounds on 

domestic animals and livestock would not expose listed species to these pesticides. Pesticide 

treatments to treat a screwworm-infested resident's dwelling and all properties within 200 meters 

of the infested resident's dwelling could cause direct and indirect effects to listed species and 

critical habitat. These effects could occur from acute toxicity to the listed species, to the prey of 

the species, or to pollinators of listed plants. APHIS will ensure that appropriate treatment 

buffers are in place from aquatic areas or habitat of listed species to so that pesticide applications 

will not adversely affect listed species, prey, or pollinators inhabiting those areas. 

b. Migratory Bird Treaty Act

The Migratory Bird Treaty Act of 1918 (MBTA), as amended (16 U.S.C. 703–712) established a 

Federal prohibition, unless permitted by regulations, to pursue, hunt, take, capture, kill, attempt 

to take, capture or kill, possess, offer for sale, sell, offer to purchase, purchase, deliver for 

shipment, ship, cause to be shipped, deliver for transportation, transport, cause to be transported, 

carry, or cause to be carried by any means whatever, receive for shipment, transportation or 

carriage, or export, at any time, or in any manner, any migratory bird or any part, nest, or egg of 

any such bird.” FWS released a final rule on November 1, 2013, identifying 1,026 birds on the 

List of Migratory Birds (FWS 2016a). Species not protected by the MBTA include nonnative 

species introduced to the United States or its territories by humans and native species that are not 

mentioned by the Canadian, Mexican, or Russian Conventions that were implemented to protect 

migratory birds (FWS 2016a).

Consumption of the sterile flies by birds should pose no risk since the flies are no different from 

wild flies except for the fact that they cannot reproduce. Because sterile release is sometimes

accomplished by small aircraft, migratory birds may be disturbed by noise and air flow changes. 

Collisions with planes may also be a risk because of the higher probability of collision at the 

lower altitudes at which small aircraft fly (Sodi 2002, Lambertucci et al. 2015). When planning 

any flights for the program, migration patterns and daily use patterns should be considered, 

especially since bird collisions can increase dramatically during migration seasons (Sodi 2002). 

Because airfields may provide locations for key bird resources such as foraging areas and nesting 

sites (Sodi 2002), special care should be taken to avoid disruption of bird activity. Flight times 

may need to be adjusted to accommodate bird activity patterns in migratory birds (Lambertucci 

et al. 2015). The program may need to defer to ground treatment in known flyways or near bird 

refuge areas. APHIS will consider specific treatment areas and plans on a case-by-case basis, 
23

taking care to locate nesting sites and flight patterns of migratory birds before implementing 

treatment or sterile release.

c. Bald and Golden Eagle Protection Act

The Bald and Golden Eagle Protection Act (16 U.S.C. 668-668c) prohibits anyone without a

permit issued by the Secretary of the Interior from "taking" bald eagles (Haliaeetus 

leucocephalus), including their parts, nests, or eggs. The Act provides criminal penalties for 

persons who "take, possess, sell, purchase, barter, offer to sell, purchase or barter, transport, 

export or import, at any time or any manner, any bald eagle ... [or any golden eagle], alive or 

dead, or any part, nest, or egg thereof." The Act defines "take" as "pursue, shoot, shoot at, 

poison, wound, kill, capture, trap, collect, molest or disturb.

Bald eagles are concentrated around water sources such as lakes, rivers, and reservoirs. Of the 

states in the affected area, Florida has the densest nesting bald eagle population, with an 

estimated 1,500 pairs. By contrast, Texas estimates its nesting pairs at about 130. The affected 

states practice a range of conservation and monitoring activities such as closure of public areas 

near nesting sites and monitoring of habitat use and breeding activity (Fish 1996, Fish 2016, 

Commission 2017, Extension No Date). APHIS will determine if bald eagle nesting sites are 

present near the vicinity of any planned treatments prior to the treatment, and determine the best 

course of action on a case by case basis.

5. Environmental Justice

Consistent with Executive Order 12898, "Federal Actions to Address Environmental Justice in 

Minority Populations and Low-Income Populations," APHIS considered the potential for 

disproportionately high and adverse human health or environmental effects on minority and low

income populations. 

Resident populations should have low exposure to the insecticides used in the program. Effective 

communication about the program will be important and will require newspaper, television, and 

online notifications in English, Spanish, and any other languages of common use in the regional 

area of program implementation. For the recent infestation of Key deer in Monroe County, 

Florida, which includes the Keys, and in Miami-Dade County, Florida, where a secondary 

outbreak was found in Homestead, Table 5 summarizes some of the pertinent demographic data.
24

Table 5. Selected demographic data for Florida counties, collected in July 2015 (Bureau 

2015)

Miami-Dade County has a higher poverty rate, is more densely populated, and has a dramatically 

higher percentage of non-English speakers than Monroe County. This dichotomy is 

representative of the differences that exist within the range of the screwworm and indicates the 

need to tailor activities and outreach associated with the cooperative screwworm eradication 

program on a case-by-case basis. In Monroe County, trained volunteers promoted public 

awareness in a door-to-door campaign in neighborhoods. In Miami-Dade County, FDACS 

officials met with growers. Public awareness was heightened with displays and outreach 

materials at local shopping centers and community websites. Outreach during an outbreak might 

also occur at retirement communities, playgrounds, and churches.

The import of animals or other livestock from countries where screwworms are established is an 

activity that is not specific to any subgroup of the population. The cooperative screwworm 

eradication program could take place at any location where an infested person or animal enters 

the country. Therefore, no disproportionate effects on such populations are anticipated as a

consequence of implementing the preferred action.

V. Potential Cumulative Impacts

Cumulative impacts are those impacts on the environment that result from the incremental 

impact of a proposed action when added to other past, present, and reasonably foreseeable future 

actions regardless of what agency (Federal or non-Federal) or person undertakes other actions. 

Cumulative impacts can result from individually minor but collectively significant actions taking 

place over a period of time.

Cumulative impacts to human health from the proposed use of permethrin, doramectin, and 

coumaphos in the cooperative screwworm eradication program are not anticipated because if 

applied as the labels prescribe, exposure to humans will be negligible. Human exposure and risk 

Monroe Miami-Dade

Total population 77,482 2.7 million

Population/mi 2 74.3 1,351.50

Percent Hispanic/Latino 22.6 67

Percent foreign-born 18 52

Percent language other than English 

spoken at home 24.1 73

Percent age 65 or older 21.3 15.6

Percent in poverty 11.3 20
25

is very low for the general public. The probability of exposure is greatest for workers who apply 

the chemicals. However, the risk to this group of the population will be negligible based on the 

low risk of the chemicals when using appropriate PPE. 

Pesticide use in other Federal programs potentially contributes to cumulative impacts with 

chemicals used in screwworm eradication. Ongoing APHIS programs in the affected area using 

pesticides include fruit fly eradication programs and the cattle fever tick eradication program. 

The fruit fly eradication program chemicals include dichlorvos, methyl eugenol, trimedlure, and 

naled. Coumaphos is used in the cattle fever tick eradication program to treat cattle for ticks that 

may vector cattle fever, and extensively in apiaries for mite control (Sanchez-Bayo and Goka 

2014). Doramectin may be administered to cattle for prevention of cattle tick fever, and to hogs 

and pigs and other animals as veterinary parasiticides. Permethrin is also used medically for 

human treatment of scabies. The implementation of the SIT program will not affect any other 

organisms except screwworm, so will not add to cumulative effects. The combined effects of 

these chemicals is likely to be very low since the individual exposure and toxicity for each 

chemical is low.

The cooperative screwworm eradication program is not expected to have cumulative ecological 

effects. Dung is an important food resource for many species of insects, particularly Diptera and 

Coleoptera, and, in turn, these insects provide food for insectivorous invertebrate and vertebrate 

predators. Insect activity in dung also aids in the important decomposition process, and the return 

of nitrogenous compounds to the soil. Chemical residues of veterinary parasiticides in livestock 

manure may be toxic to some insects, and have sublethal effects such as slowed or aberrant 

development (Wall and Beynon 2012). As mentioned in Chapter 3, Potential Environmental 

Effects, Biological Resources, repeated applications of doramectin can persist in manureamended soil for several months, and accumulate (Gil-Diaz et al. 2011). However, this is not 

expected to lead to significantly increasing concentrations in soil (FDA 1997a, b). The amounts 

of doramectin excreted in dung depends on several factors, some of which are under the control 

of farm managers, such as timing of treatment and the proportion of animals being treated at any 

one time. Overall, though, these operations are considered to be relatively low-risk (Wall and 

Beynon 2012). Doramectin use for screwworm eradication is likely to be infrequent and shortlived when conducted, given that screwworm had not previously been detected in the United 

States for 30 years.

In summary, the cumulative impacts associated with the preferred alternative when assessed in 

relation to the current baseline and past, present, and future activities constitutes a small 

incremental change to the human environment. To preserve environmental quality for the human 

population and ecological resources, the cooperative screwworm eradication program would 

minimize potentially negative cumulative impacts by following best management practices.
26

APHIS does not find that any reasonably foreseeable effects caused by the program activities 

will occur later in time or be farther removed in time.
27

VI. Listing of Agencies and Persons Consulted

U.S. Department of Agriculture

Animal and Plant Health Inspection Service

Policy and Program Development

Environmental Analysis and Documentation

4700 River Road, Unit 149

Riverdale, 20737-1238

U.S. Department of Agriculture

Animal and Plant Health Inspection Service

Veterinary Services

2150 Centre Ave Bldg B

Fort Collins, CO 80526
28

VII. References

Agriculture, C. D. o. F. a. 2016a. California Agricultural Statistics Review, 2015-2016. Page 

126.

Agriculture, C. D. o. F. a. 2016b. Screwworm. Page 1.

Agriculture, N. M. D. o. 2016c. 2015 New Mexico Agricultural Statistics. Page 70.

Alexander, J. 2006. Screwworms. JAVMA 228:357-367.

Antwi, F., and G. Reddy. 2015. Toxicological Effects of Pyrethroids on Non-target Aquatic 

Insects. Environmental Toxicology and Pharmacology 40:915-923.

Association, J. o. t. A. V. M. 2017a. In Florida, Screwworm Moves Inland from Keys.

Association, J. o. t. A. V. M. 2017b. Screwworm Infestation Kills Endangered Deer.

Association, T. S. H. No Date. Texas State Almanac: Texas Plant Life.

Baumhover, A. 2002. A Personal Account of Developing the Sterile Insect Technique to 

Eradicate the Screwworm from Curacao, Florida and the Southeastern United States. 

Florida Entomologist 85:666-673.

Board, T. W. D. 2012. Climate of Texas.

Bureau, U. S. C. 2015. Quick Facts.

Cathey, H. 1990. USDA Plant Hardiness Zones.in ARS, editor.

Chaudhury, M., S. Skoda, A. Sagel, and J. Welch. 2010. Volatiles Emitted from Eight WoundIsolated Bacteria Differentially Attract Gravid Screwworms (Diptera: Calliphoridae) to 

Oviposit. Journal of Medical Entomology 47:349-354.

Commission, F. F. a. W. C. 2017. Bald Eagle General Information.

Cork, A., and M. Hall. 2007. Development of an Odour-Baited Target for Female New World 

Screwworm, Cochliomyia hominivorax: Studies with Host Baits and Synthetic Wound 

Fluids. Medical and Veterinary Entomology 21:85-92.

Data, U. S. C. 2016. U.S. Climate Data--Arizona.

Delgado, A., M. Hennessey, and D. His. 2016. Investigation into Introduction of New World 

Screwworm into Florida Keys. Pages 1-16 in C. f. E. a. A. H. USDA-APHIS, editor.

EPA. 1996. EPA R.E.D. Facts. Pages 1-14 in P. Prevention, and Toxic Substances, editor.

EPA. 2009. Reregistration Eligibility Decision (RED) for Permethrin.

EPA. 2016. Coumaphos: Draft Human Health Risk Assessment for Registration Review.

Extension, T. A. No Date. The Texas Breeding Bird Atlas--Bald Eagle.

FAO. 2008. Screwworm.

FAO/WHO. 2006. Joint FAO/WHO Expert Committee on Food Additives 

FDA. 1997a. NADA 141-061 Dectomax® Injectable Solution--Supplemental Approval 

(September 18,1997) 

FDA. 1997b. NADA 141-095 Dectomax ® Pour-On Solution--Original Approval.

FDACS. 2016a. About Screwworm Infestation in Monroe County, Florida. Pages 1-2.

FDACS. 2016b. Florida Livestock, Dairy, and Poultry Summary.

FDACS. 2016c. Screwworm Eradication Program. Page 1.
29

Fish, A. G. a. 2016. Arizona Bald Eagle Management Program.

Fish, N. M. G. a. 1996. Bald Eagle. Wildlife Notes.

Floate, K., K. Wardhaugh, A. Boxall, and t. Sherratt. 2005. Fecal Residues of Veterinary 

Parasiticides: Nontarget Effects in the Pasture Environment. Annual Review of 

Entomology 50:153-179.

Florida, U. o. 2016. Primary Screwworm, Cochliomyia hominivorax (Coquerel) (Insecta: 

Diptera: Calliphoridae). Featured Creatures.

Foundation, A. D. 2017. Hardiness Zones.

Fresia, P., M. Lyra, A. Coronado, and A. Azeredo-Espin. 2011. Genetic Structure and 

Demographic History of New World Screwworm across its Current Geographic Range. 

Journal of Medical Entomology 48:280-290.

Fresia, P., M. Silver, T. Mastrangelo, A. De Azeredo-Espin, and M. Lyra. 2014. Applying 

Spatial Analysis of Genetic and Environmental Data to Predict Connection Corridors to 

the New World Screwworm Populations in South America. Acta Tropica 138S:S34-S41.

FWS. 2016a. Migratory Bird Treaty Act Protected Species (10.13 List).

FWS, N. K. D. R. 2016b. New World Screwworm in Key Deer at the National Key Deer Refuge.

Gil-Diaz, M., A. Perez-Sanz, M. Martin, and M. Lobo. 2011. Potential Diffusion of Doramectin 

into a Soil Amended with Female Pig Manure. A Field Experiment. Journal of 

Agricultural and Food Chemistry 59:10635-10640.

Hall, M., R. Wall, and J. Stevens. 2016. Traumatic Myiasis: a Neglected Disease in a Changing 

World. Annual Review of Entomology 61:159-176.

Hightower, B.G., A.L. Adams, and D.A. Alley. 1965. Dispersal of Released Irradiated 

Laboratory-Reared Screw-worm Flies. Journal of Economic Entomology 58:373-374. 

Hightower, B.G., J.J. O'Grady, Jr., and J.J. Garcia. 1972. Ovipositional Behavior of Wild-Type 

and Laboratory-Adapted Strains of Screwworm Flies. Environmental Entomology 1:227-

229.

IAEA/FAO. 2000. Genetic Sexing and Population Genetics of Screwworms. International 

Atomic Energy Agency, Vienna, Austria.

James, M.T. 1947. The Flies that Cause Myiasis in Man. USDA, Washington, D.C.

Johnson, R., M. Ellis, C. Mullin, and M. Frazier. 2010. Pesticides and Honey Bee Toxicity--

USA. Apidologie 41:312-331.

Knipling, E.F. 1955. Possibilities of Insect Control or Eradication Through the Use of Sexually 

Sterile Males. Journal of Economic Entomology 48:459-462.

Laake, E.W., E.C. Cushing, and H.E. Parish. 1936. Biology of the Primary Screw Worm Fly, 

Cochliomyia Americana, and a Comparison of its Stages with those of C. Macellaria. 

USDA Technical Bulletin No. 500. Washington, D.C.

Lambertucci, S., E. Shepard, and R. Wilson. 2015. Human-Wildlife Conflicts in a Crowded 

Airspace. Science 348:502-505.

Lumaret, J.-P., and F. Errouissi. 2002. Use of Antihelmintics in Herbivores and Evaluation of 

Risks for the Non Target Fauna of Pastures. Veterinary Research 33:547-562.
30

Mastrangelo, T., and J. Welch. 2012. An Overview of the Components of AW-IPM Campaigns 

against the New World Screwworm. Insects 3:930-955.

Matlock, R., and S. Skoda. 2009. Mark-Recapture Estimates of Recruitment, Survivorship and 

Population Growth Rate for the Screwworm Fly, Cochliomyia hominivorax. Medical and 

Veterinary Entomology 23:111-125.

Mayer, D.G. and M.G. Atzeni. 1993. Estimation of Dispersal Distances for Cochliomyia 

hominivorax (Diptera: Calliphoridae). Environmental Entomology 22: 368-374.

NetState. No Date. The Geography of Arizona.

Novy, J. 1991. Screwworm Control and Eradication in the Southern United States of America. 

Pages 18-27 in R. Branckaert, editor. World Animal Review. FAO, Rome.

OIE. 2013. Screwworm (Old World and New World). 

Parker, L., and J. Abatzoglou. 2016. Projected Changes in Cold Hardiness Zones and Suitable 

Overwinter Ranges of Perennial Crops over the United States. Environmental Research 

Letters 11:034001.

Pitti, A., S. Skoda, K. Kneeland, D. Berkebile, J. Molina-Ochoa, M. Chaudhury, O. Youm, and J. 

Foster. 2011. Effect of Adult Screwworm Male Size on Mating Competence. 

Southwestern Entomologist 36:47-60.

Robinson, A., M. Vreysen, J. Hendrichs, and U. Feldmann. 2009. Enabling Technologies to 

Improve Area-Wide Integrated Pest Management Programmes for the Control of ScrewWorms. Medical and Veterinary Entomology 23:1-7.

Sanchez-Bayo, F., and K. Goka. 2014. Pesticide Residues and Bees--a Risk Assessment. PLoS 

One 9:e94482.

Schweikert, K., and D. Burritt. 2012. The Organophosphate Insecticide Coumaphos Induces 

Oxidative Stress and Increases Antioxidant and Detoxification Defences in the Green 

Macroalgae Ulva pertusa. Aquatic Toxicology 122-123:86-92.

Service, U. S. F. a. W. 2016. National Key Deer Refuge Screwworm Incident Information. Page 

1.

Shleier, J., and R. Peterson. 2010. Toxicity and Risk of Permethrin and Naled to Non-target 

Insects after Adult Mosquito Management. Ecotoxicology 19:1140-1146.

Society, C. N. P. 2017. California Native Plant Society.

Sodi, N. 2002. Competition in the Air: Birds Versus Aircraft. Auk 119:587-595.

The Center for Food Security and Public Health, I. S. U. 2007. Screwworm Myiasis. Iowa State 

University.

Toynton, K., B. Luukinen, K. Buhl, and D. Stone. 2009. Permethrin General Fact Sheet National 

Pesticide Information Center, Oregon State University Extension Services.

University, N. M. S. No Date. Climate in New Mexico.

US-EPA. 2011. Pesticides: Registration Review. U.S. Environmental Protection Agency.

USDA-APHIS. 2014a. FAD PReP: Foreign Animal Disease Preparedness and Response Plan. 

Pages 1-29.

USDA-APHIS. 2014b. New World Screwworm. Pages 1-2.
31

USDA-APHIS. 2016. New World Screwworm Ready Reference Guide--Maps and Timelines. 

Pages 1-3 in USDA-APHIS, editor., Riverdale, MD.

USDA-APHIS. 2017a. New World Screwworm.

USDA-APHIS. 2017b. USDA Announces Eradication of New World Screwworm in Florida.

USDA-APHIS. 2017c. Final Report for the APHIS Veterinary Services Response to the 2016-

2017 Outbreak of New World Screwworm (NWS) in Florida. USDA-APHIS.

USDA-NASS. 2012. 2015 Releases and Publications.

USDA-NASS. 2016a. 2014 State Agriculture Overview for California. USDA-NASS.

USDA-NASS. 2016b. 2014 State Agriculture Overview for Florida. USDA-NASS.

USDA-NASS. 2016c. 2014 State Agriculture Overview for Texas. USDA-NASS.

USDA-NASS. 2016d. Arizona Agricultural Statistics 2015. Page 36.

USDA-NCRS. 2006. Land Resource Regions and Major Resource Areas of the United States, 

the Caribbean, and the Pacific Basin.

USDA-NRCS. 1999. Soil Taxonomy: A Basic System of Soil Classification for Making and 

Interpreting Soil Surveys.

USDA-NRCS. 2010. Keys to Soil Taxonomy. U.S. Department of Agriculture–Natural 

Resources Conservation Service.

USDA-VS. 2016. Situation Report. Pages 1-7 in V. Services, editor., Riverdale, Md.

Vargas-Terán, M., H.C. Hofmann, and N.E. Tweddle. 2005. Impact of Screwworm Eradication 

Programmes Using Insect Sterile Technique. Pages 629-650 in V.A. Dyck, J. Hendrichs, 

and A.S. Robinson, editors, Sterile Insect Technique: Principles and Practice in AreaWide Integrated Pest Management., Springer, Dordrecht, The Netherlands.

Wall, R., and S. Beynon. 2012. Area-Wide Impact of Macrocyclic Lactone Parasiticides in Cattle 

Dung. Medical and Veterinary Entomology 26:1-8.

Wyss, J. 2000. Screwworm Eradication in the Americas.

Zoetis. 2013a. Dectomax (Doramecin) Pour-On Antiparasitic 0.5% Pour-On Solution for Cattle.

Zoetis. 2013b. Dectomax (Doramectin) Antiparasitic 1% Injectable Solution for Cattle and 

Swine.